Add arm64 job to CI

[lewfish]

We now have the ability to build arm64 Docker images which will run natively on a Macbook M1. Ideally we would add an arm64 job to the CI matrix, and build and test on push to PRs, and publish to Quay when PRs are merged. This is currently difficult because Github Actions does not provide an arm64 runner. It's possible to add a self-hosted runner, but this will require setting up our own or cloud-hosted machines to run the jobs. Having builtin support is on the roadmap, and there is some discussion about workarounds using cloud-hosted arm64 machines.

We have some experience with setting up a self-hosted runner with a Raspberry Pi arm64 machine, but this will probably run very slowly and/or run out of memory. We can also do the arm64 run using QEMU emulation on an amd64 machine, but that is also very slow (it takes an hour just to build the image) and behaves differently in emulation. I found that an error that was "fixed" on my Macbook occurred while running in emulation on CI.

We may be able to speed things up by making our Dockerfile build faster or by using caching.

[lewfish]

PR #1536 shows how to setup the CI to test and publish an arm64 image.